The present invention relates to heat curable silicone compositions. More particularly, the present invention relates to low temperature, fast curing silicone compositions which include a rhodium catalyst.
Silicone compositions are known to cure through a variety of mechanisms. For example, moisture cure, photocure and heat cure mechanisms are commonly used. In many applications, such as in the electronics industry, the use of heat cured mechanisms has limitations due to the heat sensitivity of the electronic components. For example, in electronic sealing applications, such as the sealing of electronic module boxes containing electronic components, the use of high temperature sealants to seal the module can deleteriously affect the electronic components. For such applications, moisture curing or photocuring compositions have been found to be more appropriate. Moisture curing compositions, however, are slower to reach full cure than other mechanisms. Photocure compositions often do not have sufficient cure-through-volume (CTV) and are therefore combined with moisture curing mechanisms to ensure full cure.
Additionally, advances in the electronic industry have made thermal management an increasingly important consideration, particularly with respect to packaging issues. For instance, heat build-up in electronic products tends to reduce reliability, slow performance and reduce power-handling capabilities. There is, therefore, a desire generally to reduce power consumption of electronic components, while increasing their number on semi-conductor chips which are reduced in size. Also, chip-on-board technology, where semi-conductor chips are mounted directly to printed circuit boards, creates further demands on thermal management because of the more efficient use of the surface area, creating increased chip density.
Numerous heat curable silicone compositions are disclosed in the patent literature. Many of these compositions disclose cure temperatures which are either too high for use in electronic applications, or disclose relatively low cure temperatures (e.g. about 100xc2x0 C.) which require long cure times, which are often undesirable from a manufacturing standpoint. Moreover, such existing patent documents fail to appreciate the balancing of cure speed with stability and shelf-life of the compositions. Additionally, although various fillers are known, low temperature curing compositions have not been recognized to demonstrate controlled flow properties and viscosity, as well as low coefficients of thermal expansion, all of which are important to many electronic applications.
U.S. Pat. No. 4,444,944 to Matsushia, discloses an example of a heat-curable silicone composition which cures at relatively high temperatures. This patent discloses a heat curable, thermally conductive silicone composition having a crosslinkable polyorganosiloxane, a polyorganohydrogensiloxane crosslinking agent, an alumina powder having an average particle size in the range of 2.0xcexc to 10xcexc and an oil absorption of xe2x89xa715 mL/g, and a platinum catalyst. These compositions can be cured by heating under ambient conditions at temperatures of about 250xc2x0 to 450xc2x0 C. Other platinum-group catalysts such as rhodium, iridium, ruthenium, and osmium are disclosed as useful.
U.S. Pat. No. 5,312,885 to Takago, et al., discloses curable organosiloxane compositions which contain a vinylsilyl-terminated perfluoropolyalkylene or perfluropolyalkylene polyether compound as the reactive resin and employ specific rhodium catalysts for cure. The use of the rhodium catalysts is reported to impart greater storage stability without viscosity increases as compared to platinum-based catalysts. These compositions are directed toward stable compositions which do not use inhibitors. These compositions are disclosed as being curable when heated at temperatures from 70xc2x0 to 150xc2x0 C., though preferably at temperatures greater than 110xc2x0 C.
U.S. Pat. No. 5,008,307 to Inomata, discloses relatively low temperature curing compositions which require long cure times. Cure temperatures of 100xc2x0 C. for one hour and 80xc2x0 C. for four hours are disclosed. This patent discloses thermally conductive silicone compositions which include an organopolysiloxane capable of reaction with an organohydrogenpolysiloxane having at least two SiH bonds, two types of aluminum powders which are a mixture of different sized spherical shaped particles, and a platinum catalyst.
Platinum and rhodium catalysts are commonly recited in prior patent documents directed toward heat curing silicone compositions. Generally, such prior patent documents often disclose them in a list of useful catalysts, along with others from the platinum group of the periodic table.
U.S. Pat. No. 5,552,506 to Ebbrecht et al., discloses the production of acrylic-modified organopolysiloxanes using rhodium catalysts. The resulting reactive compounds are disclosed as being useful as radiation curable lacquers or coating compositions, or as additives in such systems. These compounds are also disclosed as being thermally curable with the addition of peroxides.
U.S. Pat. No. 5,629,399 to Juen et al., discloses room temperature curing organosiloxane compositions which have an alkenyl-containing polyorganosiloxane, an organohydrogensiloxane, a platinum catalyst, a methylvinylcyclosiloxane and an acetylenic alcohol. The platinum catalyst is generally used in amounts of 5 to 250 parts by weight of platinum metal per million parts of the combined weights of the other components. The methylvinylcyclosiloxane component is disclosed as effecting the working time of the composition and the demold time, i.e. the time between when the composition is mixed and poured into a mold and the time when the resulting elastomer can be removed from the mold without permanent deformation. The acetylenic alcohol is used in amounts of 0.002 to 0.11% by weight. This component is also disclosed as effecting the working time and the demold time of the resulting composition. Reinforcing fillers such as finally divided silica and non-reinforcing fillers such as quartz alumna, mica and calcium carbonate are also disclosed as being useful in this composition. These compositions are designed for and disclosed as being room temperature curing compositions, with longer working time and shorter demolding time.
U.S. Pat. No. 5,270,457 to Vanwert, et al. discloses one part curable compositions having a curable polyorganosiloxane containing at least two alkenyl radicals per molecule, an organohydrogensiloxane crosslinker having at least two silicone bonded hydrogen atoms per molecule, a hydrosilation catalyst chosen from the platinum-group of the periodic table, and an adhesion promoting composition consisting of essentially of an epoxy-substituted silane and a cure inhibitor, such as cyclic methylvinylsiloxane, and an acetylenic alcohol containing at least six carbon atoms. These compositions are disclosed as curing at temperatures below 100xc2x0 C.
Various applications in the electronic industry, including the sealing of electronic parts (such as underfill, glob top and dam and fill applications in microelectronic assemblies), potting of electronic parts, conformal coating applications, thermal and electrical conductive applications, as well as adhesive applications, would all benefit from silicone compositions which have the ability to rapidly cure, without exposure to high heat. Currently, among the fastest curing silicone compositions are those which use heat curing mechanisms, requiring cure temperatures too high for many electronic applications. Moreover, many electronic applications require silicone compositions which have the capability to not merely skin-over or partially cure, but fully cure in rapid fashion. Such rapid cure through volume (xe2x80x9cCTVxe2x80x9d) is currently best achieved by using high temperature curing silicone compositions.
Conventional high temperature silicone compositions additionally have the disadvantages associated with high energy consumption, inefficient manufacturing processes and the higher costs associated therewith. Thus, there is a need for a rapid, low temperature curing silicone composition, which has a commercially acceptable shelf-life and which overcomes the disadvantages with conventional heat curing silicone compositions.
The present invention provides rapid, curing silicone compositions, capable of curing in a commercially acceptable time frame at temperatures lower than have been used in the past. These compositions can be used in a variety of applications, for example, in the electronics industry (such as in underfill, glob top and dam and fill applications in circuit board assembly), sealing applications, such as the sealing of electronic modules, potting applications, conformal coatings, thermal and electrical conductive applications, as well as adhesive applications, are among those for which the inventive compositions are useful.
In one aspect of the present invention, there is provided a heat-curable silicone composition which includes:
(a) a reactive silicone having at least two unsaturated functional groups;
(b) a silicone crosslinker having at least two reactive silicon hydride functional groups;
(c) a catalyst system including a rhodium catalyst; and
(d) a stabilizingly effective amount of an inhibitor system including a peroxide and an acetylenic compound.
The rhodium-based catalyst system of the present invention has been found to reduce the cure temperature of the inventive silicone compositions, as compared to traditional catalysts used with heat cure silicone compositions such as those based on platinum. Moreover, it has further been discovered that the use of rhodium catalysts in combination with platinum catalysts gives a more rapid cure, at a lower temperature, than using platinum alone and further improves the cure-through-volume of the cured reactive product.
Thus, in another aspect of the present invention there is provided a heat-curable silicone composition which includes:
(a) a reactive silicone having at least two unsaturated functional groups;
(b) a silicone crosslinker having at least two silicon hydride functional groups; and
(c) a catalyst system including in combination a rhodium and platinum catalyst.
The present invention further includes articles of manufacture, such as chip-on-board electronic devices, which include the inventive silicone compositions as encapsulants for the electronic components. Thus, in another aspect of the invention there is provided an encapsulating composition for use on semi-conductors, which includes:
(a) a reactive silicone having at least two unsaturated functional groups;
(b) a silicone crosslinker having at least two silicon hydride functional groups;
(c) a catalytically effective amount of a catalyst system including a rhodium-based catalyst or a combination of a rhodium-based catalyst and a platinum-based catalyst;
(d) optionally a stabilizingly effective amount of an inhibitor system which includes a peroxide, an acetylenic compound and combinations thereof; and
(e) optionally a flow modification agent which includes a dense metal oxide material, such as alumina, desirably in a generally spherical shape.
In another aspect of the present invention, there is provided a method of preparing heat-curable silicone compositions which have rapid curing and high cure-through-volume, which method includes the step of combining in admixture, a composition which includes:
(a) a reactive silicone having at least two unsaturated functional groups;
(b) a silicone crosslinker having at least two silicon hydride functional groups; and
(c) a catalyst system including in combination a rhodium and platinum catalyst.
In yet another aspect of the present invention there is included a heat curable silicone which in addition to the aforementioned reactive silicone and silicone crosslinker components, further includes in combination a peroxide and an acetylenic compound as a stabilizing or inhibitor system. Further, the inventive compositions also include flow modification agents which serve to enhance or otherwise control the flowability and/or viscosity of the final composition. The flow modification agents also serve to lower the coefficient of thermal expansion of the total composition and may also provide conductivity.
The present invention further includes a method of providing a low temperature-curing, rapid seal on a fixture such as an electronic part, the steps of which include administering to a surface of a fixture compositions of the present invention set forth therein, and permitting said compositions to cure